This project is attempting to expand our understanding of the neurobehavioral profile, and the clinical syndromes, and improve our knowledge of the natural history of mtDNA point mutations. We are trying to determine whether mtDNA point mutations produce any measurable disturbance of thinking, attention, intelligence, behavior or neurological functioning. Specifically we will examine the associations between mtDNA point mutations in various tissues, metabolic indices, measures of brain pathology and clinical features. Answers to these questions should allow us to provide family members with a better understanding of potential risks for asymptomatic/oligosymptomatic relatives, and prognosis regarding the evolution of the disease process. Patients with documented mtDNA point mutations and their asymptomatic/oligosymptomatic maternal relatives will be invited to participate in this study. All study participants will undergo the same comprehensive evaluation and assessment including mtDNA point mutation analysis in blood, skin fibroblasts, urine sediment and hair follicles; and a complete neurological examination, cognitive and neurobehavioral assessment, blood studies, urine collection for organic acid measurements, a cranial magnetic resonance image (MRI)scan, and a magnetic resonance spectroscopy (MRS) using 1.5 Tesla scanner. The presence of mtDNA mutation will be quantitated in blood, cultured skin fibroblasts, urine sediment and hair follicles and compared to the lactate concentration in body fluids, cognitive and neurobehavioral profiles and cerebral MRS. We plan to enroll 25 patients per year with repeat assessment on an annual basis. Some or all of these subjects may represent members of a single family since mtDNA is inherited from the maternal lineage. Spouses and fathers of patients will be invited to participate in these studies as control subjects. The minimal age of entry into the study will be 6 years or older to insure accessibility in all domains.